Enhanced range transponder system

ABSTRACT

An identification and telemetry system including an interrogator ( 1 ) containing an interrogator antenna ( 2 ) for generating at an interrogation frequency, an interrogtion signal. The interrogation signal is adapted to excite over an electromagnetic coupling path (M 1 ) at least one coded label ( 3 ) containing a label antenna ( 4 ) and a label microcircuit ( 5 ). The coded label is adapted to extract energy from the label antenna and to generate a label reply signal. The label reply signal is adapted to be conveyed to a label reply antenna and, via an electromagnetic coupling path, to a receiver in the interrogator. The label antenna is placed in proximity to a further antenna ( 6 ) being a parasitic antenna coupled electromagnetically (M 2 , M 3 ) with the interrogator antenna and with the label antenna so as to enhance transfer of power between the interrogator and the coded label.

FIELD OF THE INVENTION

The present invention relates to a system for remote identification ofor telemetry from objects using electronically interrogatable codedlabels. In particular the invention relates to a system for automatedidentification of articles in a warehouse or in a cargo shipping systemwherein an electronic sub-system called an interrogator including atransmitter and receiver extracts by electromagnetic means usefulinformation from an electronically coded label attached to such articlesas they are processed through sorting operations or are stacked,inventoried or collected within a warehouse. Although the presentinvention is herein described with reference to warehouse operations, itis to be appreciated that it is not thereby limited to suchapplications. Thus the identification or telemetry system of the presentinvention may be applied to object identification operations generally.

BACKGROUND OF THE INVENTION

A simplified diagram of the type of system to which the inventionrelates is shown in FIG. 1. The system uses the principle ofelectromagnetic communication in which an interrogator containing atransmitter generates an electromagnetic signal which is transmitted viaan interrogator antenna to an electronic label containing a labelreceiving antenna. The label antenna may receive a proportion of thetransmitted energy, and through a rectifier may generate a DC powersupply which may be used for operation of a reply generation circuit,connected either to the label receiving antenna or to a separate labelreply antenna, with the result that an information bearingelectromagnetic reply signal is transmitted from the label back to thereceiver of the interrogator.

In the diagram of FIG. 1 the antennae within the interrogator and thelabel take the form of inductive loops. When separation between theinterrogator and the label is much less than a wavelength at theoperating frequency, the principal form of direct coupling between suchinductive loops is provided by reactive energy storage fieldssurrounding those antennae and is expressed in terms of mutualinductance depicted as M1 in FIG. 1, between those loops.

In practical installations of electronic labelling systems it isdesirable that labels be robust, easy to install and of low cost, All ofthese considerations suggest that the labels, and hence the couplingloop within the labels, be made physically small. Because somecomponents within the label may also be fragile, it is sometimesdesirable that the label be embedded well within rather than attached tothe object being identified.

All these considerations combine to make the interrogation range whichmay be achieved with such labels and within practical restraints smallerthan is desirable. This limitation is particularly relevant where theobject to be identified is large, and its placement in relation to theinterrogator is uncertain. Although the problem of limited interrogationrange of the label can be alleviated somewhat by insuring that it isplaced close to the edge of the object to be identified, therefrequently occur situations in which objects to be identified arestacked, so that other objects prevent ready access of an interrogatorantenna to an edge of the object closest to the label position.

One solution to this problem could be the use of a large antenna placedexternally to the label, and directly connected thereto. However, such asolution generally leads to labels becoming of an unacceptable cost andfragility, and being of unsuitable dimensions for attachment to avariety of objects.

SUMMARY OF THE INVENTION

In one aspect of the present invention the problems discussed in thepreceding paragraphs may be alleviated by the addition of a parasiticantenna, sometimes known as an auxiliary antenna, which while not beingdirectly connected to the label, does have some coupling thereto, andhas coupling also to the interrogator antenna. Such coupling between theinterrogator antenna and the parasitic antenna can, either as a resultof the parasitic antenna being physically large or through a portion ofit lying close to the interrogator antenna, easily exceed directcoupling between the interrogator and the label. Thus the parasiticantenna may through being of a larger size collect more energy from theinterrogation field, which may by virtue of electromagneticcompatibility regulations, be constrained to have a low value.

The parasitic antenna may in addition visit more of the external regionsof a large item to be identified, so that however that item is stacked,there is the possibility of bringing the antenna of an interrogatorsystem into reasonable proximity to the parasitic antenna. In addition,the parasitic antenna may be shaped or oriented to be responsive to morefield directions than is practicable for the label antenna itself.Moreover, the parasitic antenna may be made conformable to a range ofobjects of shape unpredictable at the time of manufacture of the labelitself, or to objects whose shapes change over time.

The parasitic antenna may also simultaneously provide enhanced couplingbetween an interrogator and a plurality of electronic labels which theinterrogator interrogates either simultaneously or within a short spaceof time.

In all of these circumstances the parasitic antenna preferably shouldhave appropriate coupling to the label antenna or the label circuits, sothat a significant portion of the energy received by the parasiticantenna may be transferred to the label, and a significant portion ofthe reply generated by the label may in turn be transmitted via theparasitic antenna back to the interrogator.

According to the present invention there is provided an identificationand telemetry system including an interrogator containing aninterrogator antenna for generating at an interrogation frequency aninterrogation signal adapted to excite over an electromagnetic couplingpath at least one coded label containing a label antenna and a labelmicrocircuit, said coded label being adapted to extract energy from saidlabel antenna and to generate a label reply signal, said label replysignal being adapted to be conveyed to a label reply antenna and via anelectromagnetic coupling path to a receiver in said interrogator,wherein said label antenna is placed in proximity to a further antennabeing a parasitic antenna coupled electromagnetically with saidinterrogator antenna and with said label antenna so as to enhancetransfer of power between said interrogator and said coded label.

According to one aspect of the present invention the coupling betweenthe further or parasitic antenna and both interrogator and label may beprovided by mutual inductance. This is a non-contact form of couplingwhich may provide significant advantages in installation, in that thereis no necessity for electrical contacts to be made or exposed, and thusthe disadvantages of such contacts which include corrosion or accidentaldamage to the label through direct electric contact to damagingpotentials may be avoided. In addition installation is simplified as thetime to make contacts may be eliminated from the installation procedure.

According to another aspect of the present invention, such couplingeither between the label and the parasitic antenna or between theparasitic antenna and the interrogator can be provided by mutualcapacitance. The latter may be created by placing elements in proximity.

Coupling between the label antenna and the parasitic antenna may befurther enhanced by shaping the parasitic antenna so that currentstherein produce enhanced electromagnetic fields in the small regionoccupied by the label antenna.

According to another aspect of the invention, the parasitic antenna canbe made resonant at the operating frequency of the interrogation system.It is particularly advantageous if this resonance could be created byappropriate shaping of the parasitic antenna to create an appropriatecombination of inductance, capacitance and flux collecting area, withoutthere being any contacts made within the parasitic antenna, and withoutany additional components, such as lumped capacitors, being addedthereto.

According to another aspect of the invention the parasitic antenna maybe shaped so that it is sensitive to field components which are in adifferent direction to those which directly excite the label antenna,and can be further configured, particularly in respect of its impedancecharacteristics, so that directly induced voltages within the labelantenna are not in phase with voltages indirectly introduced throughoperation of the parasitic antenna, so that the two coupling mechanismsmay not interfere destructively.

According to yet another aspect of the present invention, the rectifiercircuit within the label may be configured so that it is suitable formonolithic integration and at the same time requires no dc return pathto be provided either by the antenna within the label or by a parasiticantenna to which there is no direct connection.

According to a still further aspect of the present invention theparasitic antenna may be shaped to minimise its interaction with objectsat a greater distance than the proposed interrogation antenna distance,while maintaining the interaction with the interrogation antenna, sothat detuning of the parasitic antenna and resultant loss of sensitivitythrough the presence of nearby objects including ground may beminimised.

According to a still further aspect of the present invention theparasitic antenna may be constructed from conducting portions of theobject itself to be identified. As current distributions in the antennain such cases are not always satisfactory for interrogation with aparticular carrier frequency of the interrogator, the interrogator insuch cases may be made adaptive so that interrogations with a range ofcarrier frequencies may be made with a view to performing aninterrogation with a current distribution on the object which producesgood coupling between the label, object and interrogator. For suchvariable frequency interrogation the interrogation antenna itself andthe signal separation systems within the interrogator may bere-configured to enhance coupling to the antenna of the object to beidentified or to enhance separation between the interrogation and replysignals within the interrogator.

According to a further aspect of the invention signal separation systemwithin the interrogator may be configured to take an optimum form inrelation to phase noise characteristics of the interrogator transmitterand frequency dependent characteristics of the impedance of theinterrogator antenna.

According to a still further aspect of the invention the antenna of theinterrogator may be configured to minimise strength of the interrogationfield at an electromagnetic compatibility enforcement position withoutsignificant effect on the field established at the position of theobject to be identified.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now described withreference to the accompanying drawings wherein:

FIG. 1 shows major sub units of an electronic label identificationsystem containing an interrogator, an electronic label, and a parasiticantenna with one form of coupling to the label;

FIG. 2 shows an alternative electronic labelling system including aninterrogator, a label, and a parasitic antenna with a different form ofcoupling to the label;

FIG. 3 shows elements which can be combined to make several forms ofelectronic coded label, and providing for various forms of non-contactcoupling thereto;

FIG. 4 shows a configuration of an interrogator, a movable hand-heldinterrogator antenna, a deformable parasitic antenna coupled in turn toan electronic label;

FIG. 5 shows a plurality of labels and a resonant parasitic antenna withno internal connections and no direct contact with the labels;

FIG. 6 shows a label, and another form of resonant parasitic antennawith no internal contacts and making no contact with the label;

FIG. 7 shows a parasitic antenna with a large flux collecting area andenhanced coupling to a label;

FIG. 8 shows a parasitic antenna and a label oriented to sensedifferently directed components of the interrogation field;

FIG. 9 shows a rectifier circuit suitable for microcircuit integrationin a label and requiring no dc return either within the label antenna oran external parasitic antenna;

FIG. 10 shows a parasitic antenna configured to minimise its interactionwith objects at a greater distance than the proposed interrogationantenna distance, while maintaining interaction with the interrogationantenna;

FIGS. 11(a), (b) and (c) show a combination of a code responding labelintegrated into the joining clamp of an object baling strap with thestrap itself serving as a parasitic antenna;

FIGS. 12(a), (b) and (c) show an alternative arrangement of a coderesponding label integrated into the joining buckle of a baling strap;

FIG. 13 illustrates the impedance elements of a code responding labeland a parasitic antenna formed from conducting parts of an object to beidentified;

FIG. 14 is an outline diagram of an interrogator configurable foroperation at various frequencies;

FIG. 15 is a circuit diagram showing details of a form of antennare-configuration;

FIG. 16 is a circuit diagram showing a frequency dependent andre-configurable reference termination for use in signal separationsystems within an interrogator;

FIG. 17 is a circuit diagram showing a discharge protection systemuseful when external antennas are to be connected to electronic labels;and

FIG. 18 shows a variety of interrogator antenna suitable for hand heldoperation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an arrangement of an interrogator system in which aninterrogator 1 containing an interrogator antenna 2 which forms a loopwith self-inductance L1 and excites a coded label 3 containing an labelantenna 4 of self-inductance L3 over an electromagnetic coupling pathrepresented by mutual inductance M1. The label 3 contains a labelmicrocircuit (LM) which is responsible for extracting energy from labelantenna 4, generating a label reply, and conveying that label reply tothe label antenna 4, or perhaps to some separate label antenna notshown.

The label antenna 4 is also excited via an electromagnetic coupling pathprovided by the mutual inductance M2 by parasitic antenna 6 with selfinductance L2 tuned by capacitor 7 of capacitance C1. The current whichcirculates in the parasitic antenna 6 also excites the label antenna 4over an electromagnetic coupling path shown by mutual inductance M3.

In this embodiment the label antenna 4 is likely to be small and havesmall flux collecting area, but the parasitic antenna can be madephysically large so as to have a large flux collecting area, exceedingone square metre, and can therefore be well-coupled to the fieldgenerated by interrogator 1. In a warehouse application the parasiticantenna can take the form of a wire loop which surrounds the base ofeach pallet on which goods are stacked, whereas the label antenna islikely to have an area of the order of one percent of the area of theparasitic antenna.

An advantage of this antenna configuration is that however pallets arestacked, at least a portion of the parasitic antenna will lie at anaccessible edge. In this embodiment the interrogator antenna 2 isresponsible for receiving the reply from the label and can do so via therelative weak coupling path provided by M1 and the much strongercoupling path provided by mutual inductance M2.

Suitable interrogators and labels for incorporation in systems of thisnature have already been described, for example in PCT AU90/00043, PCTAU92/00143, and PCT AU92/00477 and are incorporated herein by crossreference.

An alternative preferred embodiment of the present invention is shown inFIG. 2, in which the interrogator 1 excites the label 3 byelectromagnetic fields created by transmitter antenna 8 either viadirect coupling path 9 between transmitter antenna 8 and label antenna 4or via separate electromagnetic coupling path 10 between interrogatorantenna 8 and parasitic antenna 11 which may be directly connected viacoupling capacitors 12 and 13 to the label antenna or by no-contactmeans in which equivalent capacitors are created by placement of partsof the parasitic antenna in proximity to appropriate terminals of orleads of the label 3. The parasitic antenna may be tuned in part bycapacitors 12 and 13 and in part by the input capacitance of the labelmicrocircuit 5. The reply generated by the label is conveyed either bydirect electromagnetic path 14 to a interrogator receiver antenna 15,but may be alternatively conveyed first to the parasitic antenna andfrom there via electromagnetic coupling path 16 to the interrogatorreceiver antenna 15. An advantage of this particular embodiment is thatstrong non-contact coupling between the parasitic antenna and the labelmay be established if appropriate means to create the effectivecapacitances 12 and 13 are employed.

Suitable sub-units for incorporation with the label are shown in FIG. 3.Many of the operations within the label are as disclosed in PCTAU90/00043, PCT AU92/00143, and PCT AU92/00477 but will be forconvenience discussed briefly here. In this embodiment the label antenna4 is tuned by resonating capacitor 17 to the operating frequency of theinterrogator. Rectifier circuit 18 extracts energy from theinterrogation signal and supplies it to reply code generator circuit 19which may in some embodiments also receive energy from a label battery20. The reply code generator circuit produces a reply code, generally inthe form of a binary digital modulation of either frequency or phase ofa sub-carrier, which is used either to directly modulate the labelimpedance through modulator 21 or indirectly modulate the labelimpedance by placing a variable load on the rectifier through modulator22. Coupling of the label to external fields may be provided byinductive means via label coupling loop 4, or alternatively bycapacitive means via fly-leads 23 and 24, which run externally to thebody but are connected to circuit nodes A and B shown in the diagram oralternatively via conducting pads within the label body, those padsbeing connected to nodes A and B, the said pads being placed in closeproximity to parts of the parasitic antenna.

In FIG. 4 some practical details of an interrogator, parasitic antennaand label are shown. Here the interrogator 1 has for its interrogatedantenna 2 a coil wound on a ferrite core 24 connected via flexible cable25 to the interrogator. The interrogator antenna coil is series tuned byresonating capacitor 26. The assembly of ferrite coil and capacitor isknown as a reading wand and is brought by the operator into proximity tothe parasitic antenna 6, which in this case is tuned by three separateseries resonating capacitors 27, 28 and 29 which are distributed aroundits periphery.

As the diagram of FIG. 4 shows, the parasitic antenna may be made ofirregular shape. The employment of resonating capacitors reduceselectric fields associated with parasitic antennae of significantcircumferential length. In addition, if the parasitic antenna is ofreasonably fine wire, the self inductance of the structure is stronglydependent upon the length of the circumference, which remains constantas the parasitic antenna is deformed, and while being somewhat dependentalso upon the shape of that antenna, is not strongly so, so that theparasitic antenna can remain reasonably within resonance as it isdeformed. The reduction of electric field which derives from theemployment of the plurality of series capacitors also assists inmaintaining a substantially constant resonant frequency as deformationof the antenna takes place.

In this embodiment coupling between the parasitic antenna 6 and thelabel antenna 4 is established by placing the label close to one portionof the periphery of the parasitic antenna.

One preferred embodiment of parasitic antenna 6 is illustrated togetherwith a plurality of labels 3 in FIG. 5. In this example coupling betweenthe parasitic antenna 6 and the antennas of labels 3 is provided byplacing labels 3 inside the loop of parasitic antenna 6 and preferablyin proximity with a part, such a side of the parasitic antenna. Theresonating capacitance C1 of the parasitic antenna is however notprovided by an explicitly connected capacitance element, but is providedinstead by non-contact means, by configuring the parasitic antenna sothat its ends overlap, with an appropriate spacing and for anappropriate distance, so that the appropriate capacitance 7 is therebyprovided. This embodiment has the benefit that neither connections tothe label nor connections within the parasitic antenna are required, andthe parasitic antenna can easily be made resonant, even though it is ofa length which is substantially less than a wavelength, and a large andwell-defined flux collecting area for the parasitic antenna is definedby the configuration.

An alternative embodiment of label and parasitic antenna is shown inFIG. 6. In this case capacitances 30 and 31 between the ends of theparasitic antenna 6, which contribute to its resonant condition, areestablished not by placing the ends in proximity but by exploiting thecapacitances shown as C9 and C10, which each of those ends will haveboth to one another and to ground.

Yet another preferred embodiment for the parasitic antenna is providedin FIG. 7. In this embodiment the antenna 6 is again tuned bycapacitance 7 provided by appropriate overlap of the ends. However,coupling between the parasitic antenna and the label is enhanced byshaping the parasitic antenna to have one or more turns whichimmediately surround the area occupied by the label. This configurationhas the benefit of having a large flux collecting area for the parasiticantenna and an enhanced and adjustable (through varying the number ofturns) coupling between the parasitic antenna and the label.

Yet another preferred embodiment of the parasitic antenna and the labelis provided in FIG. 8. In this embodiment the parasitic antenna iscoupled to the label via mutual coupling by capacitances 12 and 13 whichexist between fly-leads attached to the label and sections of theparasitic antenna 6 that are placed in proximity thereto. In thisembodiment the parasitic antenna is made planar and placed in a planewhich is orthogonal to that of the label 3. In this way the two antennaswhich energise the label circuit respond to different components of themagnetic field established by the interrogator. The advantage of thissystem is that interrogation of the label for a wide range ofinterrogation field orientations becomes possible. This is particularlyso when the phase relationships established by the resulting impedanceof the parasitic antenna ensure that the voltage induced within thelabel by its coupling, either inductive or capacitive, are approximatelyin quadrature with signals induced directly in the label antenna fromthe interrogator field. There is in that situation no possibility thatthe two signals will cancel for a particular orientation of label.

One preferred embodiment for the rectifier system within the electroniclabel is shown in FIG. 9. This bridge rectifier system, which isrealised from a combination of p and n channel transistors and diodesbetween the p regions and n-type substrate, has the property that therectification process is performed in a way which minimises flow ofminority carriers to either substrate or well of the cmos fabricationprocess. In this embodiment ac excitation signals either from aparasitic antenna are conveyed by capacitors 12 and 13 or from theinternal antenna 4 are conveyed by capacitors 32 and 33 to the bridgerectifier containing n-channel transistors 34 and 35, p-channeltransistors 36 and 37 and pn diodes 38 and 39. The dc output of thisrectifier is collected in reservoir capacitor 40. It may be noted thatin this bridge rectifier a return current path between the positive andnegative supply rails labelled as VSS and VDD is provided internally tothe rectifier circuit, so that no dc contact between that circuit andthe parasitic antenna is required. The use of either inductive orcapacitive coupling to that parasitic antenna is therefore feasible andremains so whether the internal antenna 4 is directly connected to therectifier circuit or is coupled thereto via the capacitors 32 and 33 asshown.

A still further preferred embodiment for the parasitic antenna 6 isshown in FIG. 10. By tracing the current paths shown in FIG. 10, andrecognising that the current traverses the capacitor 7 provided byoverlap of conductors, it can be seen that in this embodiment theantenna can be regarded as two dipolar antennas, of which the enclosedarea is confined substantially to the periphery of the object to beidentified and to a region at the centre providing coupling with thelabel.

It can be further recognised that these dipoles are of oppositepolarity, so that the field external to the object to be identified andat a significant distance therefrom is that of a magnetic quadrupole, inwhich the field amplitude diminishes as the fourth power of distancefrom the object.

An advantage of this antenna configuration is that it is, relative to asimple dipole, subject to lesser detuning caused by large objects,including ground, which may be near to the object being identified, andin a warehousing application the resonant frequency of the antenna istherefore not much affected by whether the pallet being identified isstacked on the ground or some height above ground.

In this design, despite the reduction in included area of each of thedipoles, the conductors carrying oppositely directed currents aresufficiently separated for the area between those conductors to capturethe majority of the flux which emerges from the usual design ofhand-held interrogation antenna, the field pattern of which approximatesthat of a magnetic dipole or quadrupole.

In another aspect of the invention it is recognised that the object tobe labelled is frequently much larger than the label itself, andmetallic parts of the object will have currents induced thereon by theinterrogation field. It is advantageous to make use of thesecurrent-carrying portions of the object as a parasitic antenna whichdoes not form a part of the electronic label as manufactured, butoperates in conjunction therewith to have a strong coupling to theinterrogator electromagnetic field.

This effect can be particularly advantageous if impedance associatedwith the current-carrying portion of the object and the input impedanceof the microcircuit within the label can be made resonant, so thatreactances which can impede good transfer of power from the object tothe label are tuned out or are substantially reduced, and if theinterrogation frequency is varied so that this condition is reached ormore nearly approached.

A realisation of this condition is depicted in FIGS. 11(a), (b) and (c).In this realisation, the object to be identified is for example, a woolbale or a large cardboard container which is given strength by one ormore large metal straps, one of which is shown as 42 in the figures,fastened in tension around the periphery of the object.

The two ends of the strap are clamped together by code responding clamp41 containing insulating pad 43 with surfaces roughened to provideresistance to movement when the clamp is assembled. Opposite ends of thebaling strap are held in place by conducting wedges 44 and 45, insertedinto insulated moulded body 46, but are separated electrically throughoperation of the insulating pad 43.

Insulated moulded body 46 of the clamp provides electrical insulationbut is given circumferential strength through inclusion of rectangularmetal stampings 47. Within the clamp is a code responding microcircuit48 which has a two-terminal electrical interface to which conductingleads 49 and 50 are connected and which protrude from the inner surfacesof the moulded body of the clamp.

In view of principles already outlined in this disclosure, it may beseen that is not necessary that there be within the label any dcconnection between these terminals. Excitation of the code-respondingmicrocircuit can be performed by immersing those terminals in the strongelectric field, or through the action of the parasitic antenna, formedby the conducting parts of the object being identified and to which themicrocircuit becomes connected after the clamp is assembled.

FIG. 11(b) shows steps involved in the process of assembling the clampto the object. In this process the clamp body without the conductingwedges in place may be held in an assembly tool through which one end ofthe baling strap is first passed below the insulating pad and then bentaround the object. After traversing the circumference of the object, thebaling strap is again passed through the clamp, this time above theinsulating strap. When it reaches the end of the clamp its furtherpassage will be impeded by the clamping tool which is used to drive intoplace the upper wedge 44. The strap is then tightened on the object bypulling the free end of the strap in one direction while the clampingtool and clamped end of the strap are pulled in the other. Whensufficient tension in the strap has been established, the clamping toolis operated to drive home the second wedge 45, and to cut off the freeend of the baling strap.

An alternative embodiment of these concepts is provided in FIGS. 12(a),(b) and (c). This embodiment is suited to the use of more flexible nylonstraps 52 commonly used in industry to give strength to cardboardcartons. In this embodiment the normally woven nylon straps are madeconductive through inclusion within the weave of the straps of a numberof metal threads 52a made from a material sufficiently ductile for thestraps to remain unbroken when tension is applied to them.

The strap is joined by a buckle 51 as shown in FIGS. 12(a) and (b) whichmay be made from a metal stamping 53 and then coated with an insulatingmaterial 51 a. To the buckle is attached a circuit board containing alabel microcircuit 55 attached to circuit board 54 containing conductivepaths 56 and 57 which lie exposed on one side of the buckle. The buckle,circuit board and code responding label form an integral component. Theassembly of buckle and code responding microcircuit may be tested forcorrect function by applying electric fields to exposed conductors 56and 57. When the nylon strap forming part of the object is threadedthrough the buckle, currents induced by the interrogation field on thosestraps provide coupling between the interrogator and the microcircuit.

Aspects of the coupling provided between the label and the peripheralstraps are illustrated in FIG. 13. Depending upon the ratio of the freespace wave length at the interrogation frequency to the strap length,current distribution on the strap may be either substantially uniform orsubstantially non-uniform. The strap will, acting as a parasiticantenna, have a radiation resistance 60, an internal loss resistance 61,and when the strap is not excessively long a reactance represented byinductor 59. When the strap length exceeds a half a wavelength at theoperating frequency, the reactance can become capacitive.

In FIG. 13 the elements 63 and 64 represent capacitors associated withan electric field antenna through which the label may be interrogated inthe absence of the parasitic antenna. The input impedance of thenon-antenna portion of the label is represented by effective capacitance62 and parallel resistance 65, both of which have some variation withfrequency but not a variation as extreme as that represented by thecircuit elements representing the impedance of the strap, and of itscoupling to the label antenna.

The condition for optimum power transfer between the parasitic antennaformed by the strap and the microcircuit placed within the clamp orbuckle is one of conjugate match, that is where the label and strapimpedances are complex conjugates. As it is desirable to manufacture asingle type of label, and the strap length is dependent on the objectsize, this condition of conjugate match is difficult to maintain over awide range of objects and installations, and the structure is not alwayssensitive to externally applied fields.

This difficulty may be alleviated by configuring the interrogator of thesystem to search for replies over a range of interrogation frequencies.It is generally true that straps forming a part of a large object willhave at some interrogation frequency a large radiation resistance andpossibly a large magnitude positive or negative reactance. In thissituation coupling can be improved by lowering the interrogationfrequency to a value where the strap impedance becomes inductive, sothat it may resonate the label input capacitance, and the radiationresistance becomes lower so that the quality factor of the resonantcircuit becomes higher. In such a system the diminishing tendency ofmagnetic field producing antennas to radiate may allow largerinterrogation fields to be produced, thus further improving labelexcitation. On the other hand, smaller objects may already have lowstrap reactances and resonance is likely to occur at higher frequencies.

Although interrogation at a significant number of frequencies can becontemplated, electromagnetic compatibility regulations in somejurisdictions give the frequencies 27.12 MHz, 13.56 MHz and 6.78 MHz,which are the centres of industrial scientific and medical bands, apreferred position in that interrogation fields allowable at thesefrequencies are significantly greater, in fact by 20 dB, than atneighbouring frequencies.

A block diagram of an interrogator which permits switching between apair of interrogation frequencies is shown in FIG. 14. This interrogatorcan use many of the principles outlined in PCT AU90/00043, PCTAU92/00143, and PCT AU92/00477, but is varied from them in the way to bedescribed. In this interrogator, the signal for exciting the labeloriginates in local oscillator (LO) 66, which is switchable between thetwo frequencies 6.78 MHz and 13.56 MHz. The energising signal isamplified in transmitter power amplifier (TPA) 67 and passes throughdirectional coupler or directional detector (DD) 68 with reference armterminated in reference termination circuit (RTC) 69 and then throughantenna configuration switch (ACS) 70 to the interrogation antenna 71which may take the form of a plurality of windings on a ferrite core.

Reply signals received in the form of modulated sidebands of theinterrogation signal are passed through the antenna configuration switch70 to the directional coupler or directional detector 68 to balancedmixer (BM) 72 which is fed at its local oscillator port with a portionof the original transmitter oscillation signal. In this mixer the replysignal is down-converted to a baseband signal which is amplified inreceiver baseband amplifier (RBA) 73 and sampled by digital to analogueconverter (DAC) 74 so that a sampled version of the reply signal isavailable to micro controller (MC) 75. The micro controller conductsdigital filtering and signal analysis on the sampled reply and extractsthe reply code.

As has been described in the disclosures referenced, the interrogatoroperates in a pulse mode in that the interrogator energy is supplied fora period, commonly the order of 1 ms, sufficient to extract one coherentreply from the electronic label, this excitation period being followedby a period of no transmission during which the label reply, alreadycaptured in the analog to digital conversion process, is analysed.

In the interrogator outlined in FIG. 14, the interrogation transmissionscan alternate in the carrier frequency between two values, for example13.56 and 6.78 MHz. In the period between these alternate transmissions,the antenna configuration system 70 and reply termination circuit 69 canbe re-configured so that their characteristics are appropriate for thenext interrogation transmission.

A preferred embodiment of an antenna configuration system or switch isshown in FIG. 15. In this circuit switches 76, 77 and 78 operating undercontrol of the micro controller 75, allow the self inductance ofinterrogator antenna 71 the effective values of resonating capacitor 79and 80 and damping resistors 81 and 82 to be changed so that for eachinterrogation transmission the antenna is tuned, with an appropriatequality factor, to the carrier frequency of the interrogation frequencythen in use. In this tuning the antenna configuration not only requiresthe correct resonant frequency and quality factor but also theappropriate dynamic impedance to match characteristic impedance of thedirectional coupler or directional detector 66 and any transmission lineplaced between that element.

A preferred embodiment of the receiver termination circuit is shown inFIG. 16. The important aspects of FIG. 16 are firstly that in whatevercondition the switches are found, the termination presented todirectional coupler or directional detector 68 generally employed inhomodyne interrogators using a single interrogation antenna is not afixed broad-band impedance, but that of a resonant circuit, of which thecharacteristics can be matched to that of the interrogation antenna. Asthe principal source of noise in such homodyne interrogators is phasenoise in the transmitter signal, the use of this form of terminationallows the minimisation of coupling, over the band occupied by the replysignal, of this phase noise into the receiver.

In FIG. 16 all elements of the tuned circuits are shown as adjustable,and can usefully be fine tuned during manufacture so that minimisationof transmitter signals and noise entering the receiver can be enhancedby such adjustment. In an interrogator designed to operate at twointerrogation frequencies, switches 83, 84 and 85 operating undercontrol of the microcontroller 75 allow switching of the effectivevalues of inductance provided by inductors 86 and 87, capacitanceprovided by capacitors 88 and 89 and resistance provided by resistors 90and 91 of the reference termination provided to the directional coupleror directional detector 68. Thus the condition of minimisation oftransmitter signals and noise reaching the receiver can be maintainedfor each of the two interrogation frequencies. When more interrogationfrequencies are used, more complex switching arrangements can beemployed to configure the interrogation signal generation and separationsystems appropriately.

As an alternative to operation of the interrogator at an alternatingpair of frequencies, a considerable number of distinct interrogationfrequencies can be used. In such a case, the antenna and signalseparation re-configuration can be effected through use of voltagecontrolled resistors, inductors and capacitors, with control voltagesderived from look-up tables within the interrogator. In this way a denseexploration of antenna sensitivity with frequency can be made. Thepracticability of the concept is supported by the fact that size rangeof commonly occurring warehouse items is such that the inductance ofparasitic antennas of the type described in FIGS. 11 and 12 is notlikely to vary over a range of more than four to one, and hence resonantfrequencies formed between such antennas and labels of fixed inputcapacitance will not vary over a ratio of more than two.

One of the hazards of establishing a direct connection between aparasitic antenna and a label microcircuit as has been suggested inrelation to FIGS. 11 and 12 is presented by the discharge of staticelectricity originating in other elements into the microcircuit withconsequential damage thereto. One measure to minimise this effect hasalready been incorporated into the system described in FIG. 11. Here themicrocircuit is fully embedded within the joining clamp, and the onlyelements that can in practice make contact to the terminals of themicrocircuit are the ends of the baling strap, between which ends noelectrostatic potential can exist.

Further measures to protect against such damage are outlined in FIG. 17.Here the parasitic antenna 6 does not make direct connection to portionsof the microcircuit which can sustain dc current, but is insteadconnected by current limiting capacitors 92 and 93 to a bridge rectifiercircuit (BRC) 94 the output of which is connected to reservoir capacitor40 and over-voltage limiter (OVL) 95 and reply generation circuit 96.The reservoir capacitor 40 can be sized to have a magnitudesignificantly greater than current limiting capacitors 92 and 93, withthe effect of limiting the transient discharge voltage which can appearacross critical paths of the microcircuit. Those voltages are furtherlimited by over-voltage limiter circuit 95 which has the property ofproviding a low-impedance path between supply nodes VSS and VDD whilethe voltage between those nodes exceeds a desired value.

The robustness of the circuit can be further enhanced through the use oflarge area diodes and transistors within bridge rectifier circuit 94which can for example take the form already shown in FIG. 9. Althoughthe use of large area components within this rectifier tends to increaseits input capacitance and the losses in the rectification process, suchloss to performance in the electronically coded label is more thancompensated for by the increased sensitivity of the large 20 areaparasitic antenna formed by exploiting as shown in FIGS. 11 and 12 thecurrents which flow on the object to be identified.

A preferred form of antenna for an interrogator is shown in FIG. 18.This antenna, known as a reading wand, is suitable for hand-heldoperation in which the antenna is brought by hand near to one portion ofthe parasitic antenna which excites the label.

In FIG. 18, an insulating rod 97 carries at the ends a pair of ferritecores, over which coil windings 98 and 99 are placed. The windings 98and 99 are connected in series so that the common value of current whichflows in them produces magnetic fields which are in opposition, makingoppositely directed but same magnitude magnetic dipoles, and forming afield creation structure known as a magnetic quadrupole.

The cores are separated by a distance s which is large compared with thedistance expected between the rod end and the parasitic antenna whichexcites the label, but small in relation to both the electromagneticwavelength at the interrogation frequency and the distance at whichelectromagnetic compatibility regulations are enforced, the latter beingcommonly a distance of ten metres.

To achieve a convenient driving impedance the coil is tuned by capacitor100 and loaded by resistor 101. It may be shown that the resonantcircuit so formed should have a high quality factor to enable thecreation of a large interrogation field with small power consumption.

The windings and tuning elements are driven over a flexible transmissionline shown as the co-axial line 102 around which ferrite sleeves 103 areplaced to avoid the presence of currents on the outside of the cableconnecting the interrogation antenna to the interrogator. Such currentsmake the impedance seen at the end of the cable sensitive to wandposition, and inhibit both the achievement and preservation of a highorder of interrogation and reply signal separation within theinterrogator.

A significant advantage of the interrogation antenna shown if FIG. 18 isthat when coil 99 is placed close enough to a parasitic antenna tointerrogate a label, the field distribution near to coil 99 issubstantially that of the magnetic dipole created by coil 99 alone, anddiminishes only as the inverse third power of distance. At significantdistance however, and particularly at distances large compared with thecoil separation s, the fields of the two dipoles substantially cancel,and the resulting field diminishes as the inverse fourth power ofdistance, so that only a small field at the electromagneticcompatibility enforcement distance remains. The result is that highervalues of interrogation exciting field may be legally created close tothe end of reading wand.

According to still further aspect of the invention the label may bemanufactured without a magnetic field responding antenna and coupling tothe interrogation field may be achieved through the operation of aconducting metal strap or wire of one or more turns placed around theobject to be identified and coupled to the label either by direct orcapacitive connection.

The strap may with the input impedance of the label form a resonantcircuit which may be of high quality factor and be at one of theoperating frequencies of the interrogator. Labels may be manufacturedwith a range of capacitances of which the appropriate one is selected attime of installation to facilitate this condition.

A capacitance external to the label and dependent on the strap lengthmay be introduced to bring the strap to resonance at such a frequency.Connection between the added capacitor or label may be established bydirect or capacitive connection.

In establishing a direct connection, either between the label or addedcapacitor and the strap, conductive adhesive may be used. Inestablishing a capacitive connection between these elements acapacitance may be formed between a flat strap and flat leads connectedeither to the label or added capacitor, a thin layer of insulationadhesive may be used. The added capacitance itself may be formed by theoverlap of the magnetic field antenna strap and further strap separatedtherefrom by a thin layer of insulation which may be adhesive, and theoverlap length may be adjusted to produce the desired value ofcapacitance.

Measurements of strap inductance may be performed before addition of alabel or added capacitor, so that the component values which produceresonance at the interrogation frequency may be selected. An applicationtool may perform the strap inductance measurement and dispense theappropriate label, added capacitor, or appropriate length of overlaystrap, with the appropriate capacitance.

It will be appreciated that various alterations, modifications and/oradditions may be introduced into the constructions and arrangements ofparts previously described without departing from the spirit or ambit ofthe present invention.

What is claimed is:
 1. An identification and telemetry system includingan interrogator containing an interrogator antenna for generating at aninterrogation frequency an interrogation signal adapted to excite overan electromagnetic coupling path at least one coded label containing alabel antenna and a label microcircuit, said coded label being adaptedto extract energy from said label antenna and to generate a label replysignal, said label reply signal being adapted to be conveyed to a labelreply antenna and via an electromagnetic coupling path to a receiver insaid interrogator, wherein said label antenna is placed in proximity toa further antenna being a parasitic antenna coupled electromagneticallywith said interrogator antenna and with said label antenna so as toenhance transfer of power between said interrogator and said codedlabel.
 2. An identification and telemetry system as claimed in claim 1including a plurality of coded labels which said interrogator is adaptedto interrogate substantially simultaneously.
 3. An identification andtelemetry system as claimed in claim 1 wherein said electromagneticcoupling is provided by mutual inductance.
 4. An identification andtelemetry system as claimed in claim 3 wherein said parasitic antenna isshaped so as to enhance in the region of the or each coded label anelectromagnetic field produced by said parasitic antenna.
 5. Anidentification and telemetry system as claimed in claim 4 wherein saidshaping includes one or more turns which surround a space occupied bythe or each coded label.
 6. An identification and telemetry system asclaimed in claim 1 wherein said electromagnetic coupling is provided bymutual capacitance.
 7. An identification and telemetry system as claimedin claim 1 wherein said parasitic antenna is resonant at saidinterrogation frequency.
 8. An identification and telemetry system asclaimed in claim 7 wherein said resonance is produced by appropriatelyshaping said parasitic antenna to create a combination of inductance,capacitance and flux collecting area without any contacts being madewithin said parasitic antenna and without additional components beingadded thereto.
 9. An identification and telemetry system as claimed inclaim 7 wherein a portion of capacitance required to produce resonanceof said parasitic antenna is provided by non-contacting overlap of itsconductors.
 10. An identification and telemetry system as claimed inclaim 7 wherein a portion of capacitance required to produce resonanceof said parasitic antenna is provided by capacitance to ground of itsconductors.
 11. An identification and telemetry system as claimed inclaim 1 wherein excitation of the or each label antenna provided by saidparasitic antenna is adjusted to avoid destructive interference withexcitation of the or each label antenna provided by direct couplingbetween said interrogation antenna and said label antenna.
 12. Anidentification and telemetry system as claimed in claim 11 wherein saidexcitation is adjusted by adjusting impedance characteristics of saidparasitic antenna so that directly induced voltages within the or eachlabel antenna are not in phase with voltages indirectly introducedthrough operation of said parasitic antenna.
 13. An identification andtelemetry system as claimed in claim 1 wherein there exists no dc returnpath in the parasitic antenna.
 14. An identification and telemetrysystem as claimed in claim 1 wherein there exists no dc return path inthe or each label antenna.
 15. An identification and telemetry system asclaimed in claim 1 wherein said parasitic antenna is shaped to reducethe ratio of the electromagnetic field which it produces in a far fieldregion to the electromagnetic field which it produces in a near fieldregion.
 16. An identification and telemetry system as claimed in claim 1wherein portions of an object to be labelled or its packaging form partof the parasitic antenna.
 17. An identification and telemetry system asclaimed in claim 7 wherein the interrogator is adapted to adjust theinterrogation frequency to allow for changes in the resonant frequencyof said parasitic antenna.
 18. An identification and telemetry system asclaimed in claim 17 wherein said interrogation frequency is adjusted sothat it alternates between two values.
 19. An identification andtelemetry system as claimed in claim 17 wherein said interrogationfrequency is adjusted so that it exhibits a sequence of distinctfrequencies.
 20. An identification and telemetry system as claimed inclaim 1 wherein said interrogator is configure to reduce noise signalreaching said receiver of said interrogator as a result of noise presentin said interrogation signal of said interrogator.
 21. An identificationand telemetry system as claimed in claim 20 wherein a reduction in saidnoise reaching said receiver is provided by a termination circuit havingimpedance characteristics of a resonant circuit matched to saidinterrogation antenna.
 22. An identification and telemetry system asclaimed in claim 1 wherein said interrogator antenna is configured tomaximise the ratio of the electromagnetic field produced at a labelposition to the electromagnetic field produced at an electromagneticcompatibility regulations enforcement position.
 23. An identificationand telemetry system as claimed in claim 22 wherein the interrogationantenna includes a magnetic Quadrupole antenna.
 24. An identificationand telemetry system as claimed in claim 23 wherein ferrite sleeves areplaced on a cable connecting said interrogator antenna to saidinterrogator.
 25. An identification and telemetry system as claimed inclaim 16 wherein said parasitic antenna includes a baling strapsurrounding said object and said coded label is incorporated into ajoining buckle of said strap.
 26. An identification and telemetry systemas claimed in claim 25 including means for measuring inductance of saidstrap and for dispensing an appropriate label, capacitor or length ofoverlay strap so that values which produce resonance at theinterrogation frequency are selected.
 27. An identification andtelemetry system as claimed in claim 6 wherein the coupling between saidparasitic antenna and the or each coded label is provided in a formwhich places a limit on accidental discharge of current through the oreach coded label at the time the parasitic antenna is coupled to the oreach coded label.
 28. An identification and telemetry system as claimedin claim 1 wherein impedance characteristics of said parasitic antennaand the or each label antenna are complex conjugates.